Cooling system for gas turbine stator nozzles

ABSTRACT

A cooling system for gas turbine stator nozzles, wherein each of the vanes ( 10 ) which belong to the nozzles of the said gas turbine has a concave surface ( 11 ) and an opposite convex surface ( 12 ), which co-operate in order to define the outer shape of the vane ( 10 ), and wherein the surface of the vane ( 10 ) has a plurality of cooling holes ( 13 ), at appropriate points of the surface itself of the vane ( 10 ). In this system, the cooling hole ( 17 ) relative to the outlet edge ( 16 ) of the vane ( 10 ), is provided with an intake section ( 18 ) and an outlet section ( 19 ), which are shaped such that the cooling hole ( 17 ) has a cross-section which is variable in a direction which is radial, relative to the said vane ( 10 ).

[0001] The present invention relates to a cooling system for gas turbinestator nozzles.

[0002] As is known, gas turbines are machines which consist of acompressor and a turbine with one or more stages, wherein thesecomponents are connected to one another by a rotary shaft, and wherein acombustion chamber is provided between the compressor and the turbine.

[0003] In these machines, air obtained from the outer environment issupplied to the compressor, in order to pressurise the latter.

[0004] The compressed air passes through a series of pre-mixingchambers, each of which ends in a converging portion, into each of whichan injector supplies fuel, which is mixed with the air in order to forman air-fuel mixture to be burnt.

[0005] Inside the combustion chamber there is admitted the fuel, whichis ignited by means of appropriate spark plugs, in order to give rise tocombustion, which is designed to increase the temperature and pressure,and thus the enthalpy of the gas.

[0006] Simultaneously, the compressor supplies compressed air, which ismade to pass both through the burners and through the liners of thecombustion chamber, such that the said compressed air is available inorder to feed the combustion.

[0007] Subsequently, via appropriate pipes, the high-temperature andhigh-pressure gas reaches the different stages of the turbine, whichtransforms the enthalpy of the gas into mechanical energy available to auser.

[0008] At this point, it is also known that, in order to obtain themaximum performance from a specific gas turbine, it is necessary for thetemperature of the gas to be as high as possible; however, the maximumtemperature values which can be achieved in use of the turbine arelimited by the resistance of the materials used.

[0009] In order to make more apparent the technical problems which aresolved by the present invention, a brief description is providedhereinafter of a stator of a high-pressure stage of a gas turbineaccording to the known art.

[0010] Downstream from the combustion chamber, the turbine has ahigh-pressure stator and a rotor, wherein the stator is used to feed theflow of burnt gases in suitable conditions to the intake of the rotor,and, in particular, to convey it correspondingly to the vanes of therotor blades, thus preventing the flow from meeting directly the dorsalor convex surface and the ventral or concave surface of the blades.

[0011] The stator consists of a series of stator blades, between eachpair of which a corresponding nozzle is provided.

[0012] The group of stator blades is in the form of a ring, and isconnected externally to the turbine casing, and internally to acorresponding support.

[0013] In this respect, it can be noted that a first technical problemof the stators, in particular in the case of the high-pressure stages,consists of the fact that the stator is subjected to high-pressureloads, caused by the reduction of pressure of the fluid which expands inthe stator vanes.

[0014] In addition, the stator is subjected to high temperaturegradients, caused by the flow of hot gases obtained from the combustionchamber, and by the flows of cold air which are introduced inside theturbine in order to cool the parts which are subjected to the greateststresses from the thermal point of view.

[0015] Owing to these high temperatures, the stator blades used in thehigh-pressure stage of the turbines must be cooled, and, for thispurpose, they have a surface which is correspondingly provided withholes, which are used for circulation of air inside the stator bladeitself.

[0016] However, in this context, it should be noted that the constantrequirement for increases in the performance of gas turbines makesnecessary optimisation of all the flows inside turbine engines.

[0017] In particular, since the air which is obtained from thecompression stages has been processed as described, with a considerableincrease in the thermodynamic cycle, it is advantageous for this air tobe used as far as possible for combustion instead of for coolingfunctions, which moreover is necessary in the most critical hot areas.

[0018] An important technical problem which arises in this context thusconsists of correct metering of this air in the various areas, takinginto account the fact that the amount of air required varies accordingto the functioning conditions, the age and the level of wear ordirtiness of the turbine engine and its parts, as well as to thedimensional variations of its components during the transitoryfunctioning states.

[0019] Parts which are subjected to particular stress from the thermalpoint of view are the stator nozzles, the design of which must meet thefluid mechanics requirements necessary in order to obtain a high levelof fluid mechanics efficiency of the machine.

[0020] The design must also meet the thermal requirements, in orderfirstly to limit the temperature of the metal to below a certain value,which is determined by the materials used (and can be 900° C), andsecondly to limit the temperature gradients which are present in thematerial.

[0021] In order to assist understanding of the characteristics of thepresent invention, particular reference is now made to FIG. 1, whichrepresents in longitudinal cross-section a vane 20, which belongs to anozzle of a gas turbine according to the known art.

[0022] The vane 20 has a concave or ventral surface 21, and an oppositeconvex or dorsal surface 22, which cooperate in order to define theouter shape of the vane 20.

[0023] A plurality of cooling holes 23 are also provided, shown atappropriate points on the surface of the vane 20.

[0024] These holes or slots in fact serve the purpose of cooling the endpart of the nozzle itself.

[0025] Inside the vane 20, there are also present small boxes 24 and 25,i.e. perforated plate elements which increase the coefficient of heatexchange to values which are acceptable for the current applications(3000 W/m²K).

[0026] In fact, this part of the vane of the nozzles must maintainlimited temperatures, but at the same time the consumption of relativelycold air obtained from the compressor must be limited (for example itmust be 5-10%), in order not to detract from the performance levels ofthe entire machine.

[0027] At the outlet edge 26 of the vane 20, there is also present acooling hole 27, which has an intake section 28 and an outlet section 29shown in FIG. 1.

[0028] The known art thus has the problem of a thickness of materialwhich is excessive or too great in the vicinity of the cooling hole ofthe outlet edge of the vane 20.

[0029] This quantity of material, which is indicated as 30 and 30′ inFIG. 1, generally has in its interior temperature gradients when aredifficult to eliminate, although it is possible to increase thecoefficients of local heat exchange, to take them to values which arevery high.

[0030] It should be noted however that when the intake section of theholes is enlarged at the outlet edge, there is elimination of materialwhich has high thermal gradients, but at the same time there isreduction of the speed of the cooling air, and consequently of thecoefficient of heat exchange which occurs in the holes or slots of thevane 20, on the understanding that this comparison must be made for thesame flow rate of cooling air.

[0031] This therefore shows the risk constituted by having anexcessively high temperature of the metal, in relation to the physicalproperties of the material of the nozzle.

[0032] The object of the invention is thus to provide a cooling systemfor stator nozzles of gas turbines, which makes it possible to obtainoptimum control of the temperature of the vanes of these nozzles.

[0033] Another object of the invention is to provide a cooling systemfor stator nozzles of gas turbines, which makes it possible to eliminatethe undesired temperature gradients within the vanes.

[0034] A further object of the present invention is to provide a coolingsystem for stator nozzles of gas turbines, which makes it possible toreduce the large thickness of material in the vicinity of the coolinghole of the outlet edge of the vanes.

[0035] These objects and others according to the invention are achievedby a cooling system for gas turbine stator nozzles, which is applicableto the vanes which belong to the nozzles of a gas turbine, wherein eachof the said vanes has a concave surface and an opposite, convex surface,which co-operate in order to define the outer shape of the vane, andwherein the surface of the said vane has a plurality of cooling holes,at appropriate points of the surface of the said vane, characterised inthat the cooling hole, relative to the outlet edge of the said vane, isprovided with an intake section and an outlet section, which are shapedsuch that the cooling hole has a cross-section which is variable in adirection which is radial, relative to the said vane.

[0036] According to a preferred embodiment of the present invention, theheight of the intake section (Hin in FIG. 4), along a radial directionof the vane, of the cooling hole of the outlet edge of the vane, is lessthan the relative height of the outlet section (Hout in FIG. 3).

[0037] According to a preferred embodiment of the present invention,inside the said vane there are present undulating elements, in order toincrease the coefficient of heat exchange of the said vane.

[0038] The system according to the invention has high coefficients ofheat exchange along the entire cooling hole, and the absence oftemperature gradients inside the metal of the said vane.

[0039] According to the invention, the cooling system of the nozzles hasa plurality of elements for creation of turbulence along the walls ofthe holes themselves, in order always to guarantee a high value of thecoefficient of heat exchange.

[0040] In addition, the cooling system of the nozzles has a low loss ofload, which is localised to the mouth of the said hole, such as to avoidwasting part of the total pressure of the adjustment air in this area,leaving the cooling fluid more energy to overcome the loss of load ofthe cooling holes and of the elements for creation of turbulence.

[0041] Finally, it should be noted that the geometry of the said hole issuch as to facilitate intake of the molten alloy during casting of thesaid vane.

[0042] Further characteristics of the invention are defined in the otherclaims attached to the present application.

[0043] The characteristics and advantages of the present invention willbecome more apparent from the following description of a typicalembodiment provided by way of non-limiting example, with reference tothe attached schematic drawings, in which:

[0044]FIG. 1 represents schematically, in longitudinal cross-section, avane which belongs to a nozzle of a gas turbine, according to the knownart;

[0045]FIG. 2 on the other hand represents in longitudinal cross-sectiona vane which belongs to a nozzle of a gas turbine, according to thepresent invention;

[0046]FIG. 3 represents in radial cross-section the output section ofthe cooling holes of a nozzle of a gas turbine, according to the presentinvention; and

[0047]FIG. 4 represents in radial cross-section the input section of thecooling holes of a nozzle of a gas turbine, according to the presentinvention.

[0048] In the present description, “radial direction” refers inparticular to a direction perpendicular to the flow of gas which expandsin the machine.

[0049] In some cases, the direction of the flow of gas is also thedirection of the main axis of the machine.

[0050] With particular reference above all to FIG. 2, this figure showsin longitudinal cross-section a vane, indicated globally by thereference number 10, which belongs to a nozzle of a gas turbine,according to the present invention.

[0051] The shape of the vane 10 is particularly designed to provide therequired aerodynamic properties with reference to the gases which areprocessed by the turbine, and has a concave or dorsal surface 11, and anopposite, convex or ventral surface 12, which co-operate in order todefine the outer shape of the vane 10.

[0052] There are also provided a plurality of cooling holes 13, whichare present at appropriate points of the surface of the vane 10.

[0053] Inside the vane 10, there are also present small boxes 14 and 15,i.e. perforated plate elements which increase the coefficient of heatexchange to values which are acceptable for the current applications.

[0054] Of particular importance for the purposes of the presentinvention is the output edge 16 of the vane 10, inside which there isprovided a cooling hole 17, which has an intake section 18 which isenlarged compared with the known art.

[0055]FIG. 2 also shows the outlet section 19 of the cooling hole 17, inthe part in which the vane 10 becomes thinner.

[0056] Consequently, with this configuration, an enlargement of theintake section 18 of the cooling holes 17 of the vanes 10 is obtained.

[0057] In order to eliminate this disadvantage, the cooling holes, whichusually have a constant cross-section, can have a height which isvariable in the radial direction.

[0058] In fact, if the intake of the cooling hole is wider (area 18 inFIG. 2) in the plane in the figure, the dimension at right-angles to theplane itself (radial direction for the machine) can be smaller than inthe conventional applications.

[0059] In fact, the intake section 18 of the cooling hole 17 of theoutlet edge 16 of the vane 10 has a dimension (indicated as Hin in FIG.4) which is smaller than the corresponding dimension (indicated as Houtin FIG. 3) of the outlet section 19.

[0060] If the cooling system for the nozzle, according to the inventionin question, is also characterised by having the same dimension of thecooling hole in the vicinity of the output edge of the vane (area 29 inFIG. 1 and area 19 in FIG. 2), this will assume a purelythree-dimensional form, with the intake section 18 and the outletsection 19 indicated in FIGS. 3-4.

[0061] By means of this geometry it is therefore possible to have highcoefficients of heat exchange along the entire cooling hole 17, thuseliminating the temperature gradients inside the metal of the vane.

[0062] A further improvement of the heat exchange can also be achievedby using elements for creation of turbulence along the walls of theholes themselves, in order always to guarantee a high value of thecoefficient of heat exchange.

[0063] An additional advantage of the invention is represented by thereduced loss of load localised at the mouth of the hole, which makes itpossible not to waste part of the total pressure of the adjustment airin this area, thus leaving the cooling fluid more energy in order toovercome the loss of load of the cooling holes and of the elements forcreation of turbulence.

[0064] Another advantage of the invention occurs during casting of thevane, wherein the geometry in question forms a type of funnel in themouth area of the slots, which facilitates the intake of the moltenalloy.

[0065] The theoretical and experimental results of the present inventionhave been so satisfactory that the system can be used for new gasturbines which are widely available.

[0066] The description provided makes apparent the characteristics andadvantages of the cooling system for gas turbine stator nozzles,according to the present invention.

[0067] The following concluding comments and observations are now made,such as to define the said advantages more clearly and accurately.

[0068] The object of the solution proposed is to reduce the largethickness of material in the vicinity of the cooling hole of the outletedge of the vane.

[0069] The present invention thus consists of eliminating the said areasof large thickness of material, at the same time also eliminating thecorresponding temperature gradients.

[0070] This gives rise to the advantageous consequences previouslyillustrated with reference to the reduced loss of load localised at themouth of the hole 17, in order to avoid wasting part of the totalpressure of the adjustment air in this particularly critical area.

[0071] The geometry of the hole 17 is such as to facilitate the intakeof the molten alloy during casting of the vane 10.

[0072] Finally, it is apparent that many other variations can be made tothe cooling system for gas turbine stator nozzles which is the subjectof the present invention, without departing from the principles ofnovelty which are inherent in the inventive concept.

[0073] It is also apparent that in the practical embodiment of theinvention, any materials, dimensions and forms can be used according torequirements, and the components themselves can be replaced by othercomponents which are technically equivalent.

[0074] The scope of the prevent invention is defined by the attachedclaims.

1. Cooling system for gas turbine stator nozzles, which can be appliedto each vane (10) which belongs to a nozzle of a gas turbine, whereineach of the said vanes (10) has a concave surface (11) and an oppositeconvex surface (12), which co-operate in order to define the outer shapeof the vane (10), and wherein the surface of the said vane (10) has aplurality of cooling holes (13), at appropriate points of the surfaceitself of the said vane (10), characterised in that the cooling hole(17) relative to the outlet edge (16) of the said vane (10), is providedwith an intake section (18) and an outlet section (19), which are shapedsuch that the cooling hole (17) has a cross-section which is variable ina radial direction.
 2. Cooling system for the nozzles, according toclaim 1, characterised in that the height of the intake section (18),along a radial direction of the said vane (10), of the said cooling hole(17) of the outlet edge (16) of the vane (10), is less than the relativeheight of the outlet section (19).
 3. Cooling system for the nozzles,according to claim 1 or claim 2, characterised in that, inside the saidvane (10) there are present perforated plate elements (14, 15), in orderto increase the coefficient of heat exchange of the said vane (10). 4.Cooling system for the nozzles, according to one or more of thepreceding claims, characterised in that it has high coefficients of heatexchange along the entire cooling hole (17), and the absence oftemperature gradients within the metal of the said sheet (10). 5.Cooling system for the nozzles, according to one or more of thepreceding claims, characterised in that it has a plurality of elementsfor creation of turbulence along the walls of the holes themselves, inorder always to guarantee a high value of the coefficient of heatexchange.
 6. Cooling system for the nozzles, according to one or more ofthe preceding claims, characterised in that it has a reduced loss ofload localised at the mouth of the said hole (17), such as to avoidwasting part of the total pressure of the adjustment air in this area,leaving the cooling fluid more energy to overcome the loss of load ofthe cooling holes and of the elements for creation of turbulence. 7.Cooling system for the nozzles, according to one or more of thepreceding claims, characterised in that the geometry of the said hole(17) is such as to facilitate intake of the molten alloy during castingof the said vane (10).
 8. Cooling system for gas turbine stator nozzles,all substantially as described and claimed, and for the purposesspecified.